This invention relates to an optical disc on or from which information signals are recorded/reproduced on irradiation of a laser light beam, and an optical disc substrate used therefor.
Among known optical discs capable of recording/reproducing information signals by the user, there are a phase-change type optical disc, having a layer of a phase-change type material as a recording layer, and a magneto-optical disc having a thin film of a rare earth metal-transition metal alloy as a recording layer.
Specifically, such optical disc is configured so that a dielectric layer, a reflective layer and a protective layer are formed along with the recording layer on a transparent substrate for controlling optical properties and for assuring durability.
On such optical disc, the optical properties of minute-sized domains of the recording layer are changed by irradiation of a laser light for generating a number of pits, and information signals are recorded as a pattern of these pits. Such recording is carried out along numerous guide grooves of different diameters formed on substantially concentric circles on a major surface of the transparent substrate. That is, tracking servo of the laser spots is carried out along the guide grooves as indices in order to permit recording/reproduction to occur at correct positions on the disc.
Referring to FIG. 1, showing the cross-section of the transparent substrate, the guide grooves 50 are formed at a preset pitch on a major surface of a transparent substrate 51. The flat surface within the guide groove 50 is termed a groove or a bottom surface 52, while the flat surface corresponding to the hillock between two neighboring guide grooves 50 is termed a land or a top surface 53. The in formation signals are usually recorded on the bottom surface 52 or on the top surface 53. The former recording system is termed an in-groove recording system, while the latter recording system is termed an on-land recording system.
An inclined surface 54 between the bottom surface 52 and the top surface 53 has an inclination angle of 40 to 60xc2x0 and a width e of approximately 0.1 xcexcm. This inclined surface operates as a heat-insulating surface and thus operates for prohibiting the spreading apart of the pit.
That is, the transparent substrate 51 has the bottom surfaces 52 representing the flat surface portion in the groove 50, the top surface 53 representing the flat surface of the hillock and the inclined surface 54 between the bottom surface 52 and the top surface 53. With the in-groove recording system, a width a of the bottom surface 52 represents the recording track width, and a total width (e+b+e) of the inclined surface 54, top surface 53 and the inclined surface 54 between the neighboring bottom surfaces 52 represents a width c of a heat insulating region, as shown in FIG. 1. On the other hand, with the on-land recording system, a width b of the top surface 53 represents the recording track width, and a total width (e+a+e) of the inclined surface 54, bottom surface 52 and the inclined surface 54 between the neighboring top surfaces 53 represents a width of a heat insulating region, as shown in FIG. 1.
With the optical disc, as with other recording media, there is a demand for a still higher recording density for information signals. The recording density on the optical disc may be classed into that along the length of the tracks and that in a direction perpendicular to the track. The recording density along the track length can be raised by reducing the diameter of the laser light beam spot for shortening the pit length. On the other hand, the recording density in the direction perpendicular to the track can be raised by reducing the track pitch.
However, if, with the above-described in-groove recording system or on-land recording system, the track pitch is narrowed to a value lesser than the value currently employed, such as 0.7 xcexcm or less, interference from pits formed on a neighboring recording track is liable to be incurred, as a result of which cross-talk exceeding a prescribed value of xe2x88x9226 dB, may be produced in the playback signal.
FIGS. 2A, 2B schematically show an optical disc on which recording pits, which are amorphous pits, are formed in accordance with the in-groove recording system.
FIG. 2A is a cross-sectional view of a transparent substrate, and FIG. 2B is a top plan view of the disc with the playback light being shown converged on the groove 52. The width a of the bottom surface 52 and the width b of the top surface 53 are both 0.25 xcexcm, and the width e of the inclined surface 54 is 0.1 xcexcm while the track pitch p is 0.7 xcexcm. The playback laser light has a wavelength of 680 nm, while the numerical aperture NA of an objective lens is 0.6 so that the spot diameter is 1.38 xcexcm. The Gaussian distribution of the light intensity of the playback light spot is shown superimposed on the plan view of the optical disc.
If pits are recorded with a narrow track pitch of 0.7 xcexcm in accordance with the in-groove recording system, a portion of a pit M formed on a bottom surface 52 neighboring to an other bottom surface 52 being reproduced is intruded into a playback light spot SP thus producing interference. In addition, since the spatial frequency in the direction perpendicular to the track approaches to a limit of resolution, the tracking error signal amplitude is significantly lowered thus rendering it difficult to perform stable tracking servo, thus resulting in cross-talk.
With the on-land recording system, cross-talk is increased if the track pitch 2 is narrowed to a value of the order of 0.7 xcexcm, for basically the same reason.
Recently, a land-and-groove recording system of recording on both the top surface and on the bottom surface of the groove has been proposed as a recording system of suppressing interference from neighboring tracks.
FIGS. 3A and 3B schematically show an optical disc on which pits have been formed in accordance with the land-and-groove recording system.
FIG. 3A is a cross-sectional view of a transparent substrate, and FIG. 3B is a top plan view of the disc with the playback light being shown converged on the top surface 53. The width a of the bottom surface 52 and the width b of the top surface 53 are both 0.6 xcexcm, and the width e of the inclined surface 54 is 0.1 xcexcm while the track pitch p is 0.7 xcexcm. Since both the bottom surface 52 and the top surface 53 are used as recording tracks, the inclined surface 54 disposed therebetween acts as a heat insulating region. As in FIG. 2B, the Gaussian distribution of the light intensity of the playback light spot is shown superimposed on the plan view of the optical disc.
With the land-and-groove recording system, a height difference d between the bottom surface 52 and the top surface 53 and a playback light wavelength xcex within the substrate are controlled for satisfying the relation d=xcex/6, with the reflectivity of the recording pit M being e.g., 0%.
With the land-and-groove recording system, since the neighboring recording tracks are the bottom surface 52 and the top surface 53 having a different height level, a tacking error signal is produced with a larger amplitude, such that stable tracking servo may be achieved. On the other hand, since it is the reflectivity of the recording pit M and the height difference d between the bottom surface 52 and the top surface 53 that are controlled, reproduction of the top surface 53 is less liable to be affected by interference from the pit M recorded on the neighboring bottom surface 52. The result is that intrusion of the neighboring pit M into the playback light spot SP to the extent as shown in FIG. 3B raises no particular problem, such that the tracking density approximately twice that with the conventional recording system may be achieved.
However, with the land-and groove recording system, the problem of cross-talk is similarly raised if the track pitch is reduced to a level as low as 0.55 xcexcm.
In FIGS. 4A and 4B, the width a of the bottom surface 52 and the width b of the top surface 53 are both 0.45 xcexcm, and the width e of the inclined surface 54, that is the width of the heat insulating region, is 0.1 xcexcm and the track pitch 2 is 0.55 xcexcm. If the track pitch p is 0.55 xcexcm, the pit M recorded on the neighboring bottom surface 52 is intruded into a region of high light intensity within the light spot Sp of the playback light condensed on the top surface 53. If the neighboring pit M is intruded to a larger extent into the playback light spot SP, the light spot is similarly affected by the pit M even with the land-and groove system, thus resulting in increased cross-talk. On the other hand, since the depth of the bottom surface 52 is minutely controlled and hence the margin of tolerance in power control for groove cutting, detracking or skew or bit width margin is small, the possible errors in these factors, if any, tend to increase the cross-talk significantly.
It is therefore an object of the present invention to provide an optical disc in which the track pitch is diminished while the cross-talk is suppressed to a smaller value.
According to the present invention, there is provided an optical disc having a plurality of top surfaces, a plurality of bottom surfaces, and a plurality of portions formed on a major surface of a substrate for separating the top and bottom surfaces from each other. A recording track is constituted by a top surface and a bottom surface each having a first width. Each portion has a second width. The recording track has a track pitch P defined by the sum of the first width and the second pitch, with P less than xcfx86/2 and 0.1 xcexcm less than second width less than xcfx86/2.5, where xcfx86 denotes the spot diameter of the playback light.
Preferably, the second width is such that 0.15 xcexcm less than second width xc2x1xcfx86/2.5.
Preferably, a layer of a material exhibiting a magneto-optical effect or a layer of a phase change material is recorded as a recording layer on said substrate.
Preferably, a sub-groove having a depth not exceeding the depth of the groove is formed in the above portion.
Preferably, a raised portion having a height exceeding the height of the top surface is formed in said portion.
Preferably, the level difference between the raised portion and the bottom surface is n*xcfx86/2, where n is a natural number and xcfx86 is the playback light wavelength within the substrate.
Also preferably, the level difference between the raised portion and the top surface is n*xcfx86/2, where n is a natural number and xcfx86 is the playback light wavelength within the substrate.
In other words, the present invention provides an optical disc having an optical disc substrate having a top surface and a bottom surface making up a recording track. The track pitch P of the recording track plus the distance between the recording tracks, that is the width C of the heat insulating region, is set so that the conditions P less than xcfx86/2 and 0.1 xcexcm less than C less than xcfx86/2.5 are met, where xcfx86 is the spot diameter of the spot diameter of the playback light. The optical disc employing such optical disc substrate is less susceptible to interference by pits formed in the neighboring recording track even if the track pitch is reduced to a small value of e.g., 0.55 xcexcm, so that signal reproduction with only little cross-talk may be achieved.
If the widths of the top and bottom surfaces making up the recording track are reduced in this manner to a small value, such as 0.55 xcexcm, the pits are prohibited from being enlarged in area and well-defined pits may be formed. These well-defined pits are not only satisfactory as signals, but also enable reliable signal erasure while assuring a high erasure ratio.
In sum, with the optical disc substrate of the present invention, the track pitch P and the distance C between the recording tracks are controlled, so that, with the optical disc employing the optical disc substrate, the cross-talk can be suppressed to a small value even if the track pitch is reduced to a small value on the order of 0.55 xcexcm, thus assuring satisfactory signal reproduction and enabling the recording density to be significantly raised in the direction perpendicular to the track direction.